In a drilling rig, it is known to suspend a top drive from a travelling block by using a becket carried at the lower end of the travelling block to support the top drive via a second becket carried at the upper end of the top drive.
FIGS. 1 and 2 show a conventional travelling block 10 in which a plurality of pulley sheaves 12 are mounted on a horizontal shaft 14 supported between two side walls 16 of a housing 18. Below the housing 18 of the travelling block 10, a becket supporting framework 19 features two support plates 20, 22 of matching shape that depend downwardly away from the housing 18 in spaced apart vertical planes, each lying parallel to the shaft 14 and perpendicular to the side walls 16 of the housing. A lower end of each plate 20, 22 has an inverted U-shape, with an empty slot 24 running vertically upward between two legs. The two plates 20, 22 align with one another, whereby their aligned slots 24 define boundaries of an opening 25 passing horizontally through the becket supporting framework 19 between them in a direction perpendicular to the pulley shaft 14.
A u-shaped or arcuate becket 26 has a first end 26a pinned in place between a first leg 20a of the first support plate 20 and the matching first leg 22a of the other support plate 22. In the context of drilling rigs in the oil and gas industry, the becket is sometimes also referred to as a hook or clevis. In the closed position of the becket 26, shown in FIGS. 1 and 2, its second end 26b is likewise pinned in place between the second leg 20b of the first support plate 20 and the matching second leg 22b of the second support plate 22. With both ends 26a, 26b of the becket 26 pinned to the legs of the support plates 20, 22 by respective pins 28, 30, the becket 26 is in a closed position, in which the upward facing saddle 26c of the becket 26 closes off the bottom end of the opening 25 in a space 32 between the support plates 20, 22. To open the becket 26, one of the pins 30 is removed, thereby allowing the released end of the becket to swing downwardly about the axis of the other pin 28 under gravitational action, thereby opening up the lower end of the opening 25.
With the travelling block becket 26 in the open position, the peak of a top drive becket at the upper end of a top drive (not shown) can be lifted up into the opening 25, at which point the travelling block becket 26 is re-closed under the peak of the top drive becket. At this point, the peak of the top drive becket hooks over the saddle 26c of the travelling block becket, thereby hanging the top drive from the travelling block 10. To disconnect the top drive, the travelling block becket 26 is re-opened to enable withdrawal of the top drive becket from the travelling block becket and open space of the travelling block becket support plates.
For the safety of personnel during such opening of the travelling block becket 26 to connect or disconnect a top drive or other piece of equipment, the end 26b of the travelling block becket 26 being opened must be temporarily supported while being unpinned, and carefully lowered into the open position. Prior solutions for such support include the use of a sling with looped ends. One looped end of the sling is placed inside the opening 25, and a pinch bar is passed through the eye of this first looped end of the sling and inserted into an opening in the support framework 19 near the first pin 28. The sling is fed through the space 32 between the support legs 20a, 22a that carry the first end 26a of the travelling block becket on the first pin 28, and then is passed along the convexly curved underside 26d of the travelling block becket 26 to the opposing side of the travelling block, where the second end of the sling is pulled up past the second end 26b of the becket. Here, a lift cable from a hoist or air tugger is connected to the second looped end of the sling. The hoist or air tugger is used to pull upwardly on the second end of the sling, thus pulling the sling snugly up against the underside 26d of the becket 26. The sling thus cradles the becket 26 in place during the removal of the pin 30 from the second end 26b of the becket 26. Unpinned, this end can 26b then be lowered in a controlled manner by gradually letting out more of the lift cable so that the becket is carefully lowered in the open position in the controlled embrace of the sling.
However, there is a desire for improved safety in this operation, as the pinch bar can fall out of the looped end of the sling, in which case the becket quickly falls open in an unconstrained manner, and the pinch bar creates an overhead falling hazard to personnel below.
Applicant has developed an improved device for sling-based temporary support of the travelling becket, which may be used not only for drilling rigs, but for other applications in which a travelling block is employed.